Capturing critical gem-diol intermediates and hydride transfer for anodic hydrogen production from 5-hydroxymethylfurfural

The non-classical anodic H2 production from 5-hydroxymethylfurfural (HMF) is very appealing for energy-saving H2 production with value-added chemical conversion due to the low working potential (~0.1 V vs RHE). However, the reaction mechanism is still not clear due to the lack of direct evidence for the critical intermediates. Herein, the detailed mechanisms are explored in-depth using in situ Raman and Infrared spectroscopy, isotope tracking, and density functional theory calculations. The HMF is observed to form two unique inter-convertible gem-diol intermediates in an alkaline medium: 5-(Dihydroxymethyl)furan-2-methanol anion (DHMFM−) and dianion (DHMFM2−). The DHMFM2− is easily oxidized to produce H2 via H− transfer, whereas the DHMFM− is readily oxidized to produce H2O via H+ transfer. The increases in potential considerably facilitate the DHMFM− oxidation rate, shifting the DHMFM− ↔ DHMFM2− equilibrium towards DHMFM− and therefore diminishing anodic H2 production until it terminates. This work captures the critical intermediate DHMFM2− leading to hydrogen production from aldehyde, unraveling a key point for designing higher performing systems.

Fu et al. captured two inter-convertible gem-diol intermediates using in situ Raman spectroscopy, isotope tracking and DFT calculations.The experimental procedure is reasonable and the results could convey a conclusive massage for the mechanistic insight.However, some conclusions are too subjective, and some scientific problems require further revision.
1. SEM and TEM images are too blurred.
2. Figure 1h, counting the error, why is there a partial FE higher than 100%?3. How do the authors distinguish the FE of HMFMD2-and HMFMD-oxidation pathways?Is it by hydrogen production rate?The vibration peaks of DHMFM2--Au13 (1024 cm-1) are also present in Figure 3a-b, so the oxidation of HMFMD2-still exists at the potential of 0.53-0.93V. Therefore, the absence of H2 production at 0.53-0.93V may be attributed to the higher potential and cannot be fully considered as the effect of intermediates.4. In page 9, line 203, the oxidation peaks are shown in Figure 1d, not Figure 1c.Furthermore, there are many mistakes in detail; please check the whole manuscript carefully.5.It is recommended to add in situ IR to make the results more convincing.
Reviewer #2 (Remarks to the Author): In this manuscript, the authors studied the mechanism of 5-Hydroxymethylfurfural electrooxidation on gold for anodic hydrogen generation with Raman, GC, HPLC, DFT as well as DEMS.A lot of work has been done.However, there are some flaws in the manuscript as shown below.I would suggest reconsidering to publish the manuscript after a major revision.
1.For the DEMS measurements, I did not find the experiment's details in the manuscript.
2. Lines 297-303, typos, the mass charge ratio should be m/z or m/e instead of m/s.

Figures 2c, S9 and S10a
, the authors claimed that 0.33 V vs 0.17 V (vs.RHE) was used.However, in both Figures S9 and S10, the applied potentials were the same between 0.37 and 0.13 V, but the mass spectra signals were quite different.At positive potentials, there should be positive currents for oxidation reactions.However, the currents showed in Figures were negative.Did you purge solutions with N2 or Ar? 4. Lines 301-303, at 0.17 V (vs.RHE), the hydrogen evolution reaction on gold cannot happen at this positive potential.5. Again, in Figure S1, the currents were negative in both scan directions.You might have had oxygen in the solution.6.It is well known that aldehyde electrooxidation on Ib metals such as Au, Ag and Cu in alkaline media generates hydrogen, and has been studied for many decades.There are many relevant papers that may be helpful for mechanism study, however, were not cited by the authors.
Reviewer #3 (Remarks to the Author): In the submitted manuscript, the authors attempted to decipher the anodic hydrogen production mechanism during HMF electro-oxidation to HMFCA on gold in basic solutions.In such conditions, HMF, DHMFM, DHMFM-, and DHMFM2-are present in the bulk state.Previous studies suggested that the anodic H2 production probably originated from the aldehyde group.In this study, anodic H2 production is directly linked to the DHMFM2-, and specifically the Au-H bond formation at low overpotentials.Also, the DHMFM-↔ DHMFM2-equilibrium dictates the extent of anodic H2 production.
The proposed mechanism is reasonable but I would not consider the level of experimental and theoretical support fully sufficient as this is the main driving point of the paper.Several suggestions to improve and questions are below.
Facet dependence -how does the surface facet influence the Au-O modes and positions in the Raman spectra.Would this also cause different Au-O vibrations as well as different intermediates?
Could the Raman spectra and hypothesis be verified with metals like Au, Ag, and Pd which have recently been shown to be close to 100% selective for the anodic pathway that produces H2?I would assume then, if the mechanism proposed in the paper is correct, that the DHMFM2 would be the main intermediate detected in the Raman spectrum.Luckily these materials are also SERS active.This would also help to determine if the proposed insights are generalizable.
It seems Figure S11 is mistakenly referred as S12 in line 477.Please double check all the figures.How thick is the Au catalyst on the Ni.Even if Ni is not active for this, are there any interfacial effects or synergy in a way that Ni augments the activity of Au?This could be verified by depositing the Au on a surface like carbon paper and testing performance.
For Fig. S11 -H2 production should also be measured.
In the abstract it is mentioned that "The main challenge lies in the rapid electrocatalysts' performance decay, which necessitates a comprehensive understanding of the reaction mechanism to develop novel electrocatalysts."Please mention that how this proposed mechanism can be used to address the electrocatalyst stability issue.Is this different than previous systems that were reasonably stable for the anodic H2 production/aldehyde production pathway?

Response Letter to Referees
Manuscript ID: NCOMMS-23-05443 Dear editor and referees, We sincerely thank all the reviewers and editor for the valuable comments and suggestions.In this Response Letter to Referees, we have addressed and clarified all the comments/concerns raised by the reviewers.With substantial amount of work having been added into the revised manuscript according to the reviewers' comments, the quality of our manuscript has been significantly improved and we hope that the revised manuscript can now meet the high standards of Nature Communications and be published in your prestigious journal.
Listed below are our point-by-point responses to the reviewers' comments, and all the related revision are highlighted in the revised manuscript.

REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): Fu et al. captured two inter-convertible gem-diol intermediates using in situ Raman spectroscopy, isotope tracking and DFT calculations.The experimental procedure is reasonable and the results could convey a conclusive massage for the mechanistic insight.However, some conclusions are too subjective, and some scientific problems require further revision.
Reply: Thanks for the comments.Following the suggestions, we have made extensive revisions to correct the scientific problems, and conducted a series of experiments, such as in situ IR and Raman, which more convincingly prove the proposed mechanisms and further confirm our conclusion.1h, counting the error, why is there a partial FE higher than 100%?

Figure
Reply: Thanks for the comment.The partial FE of higher than 100% is due to a measurement error in the quantity of HMFCA produced through non-Faradaic processes.In this electrochemical system, there are two Faradaic processes, i.e., HMFMD 2˗ and HMFMD ˗ oxidation pathways.However, there exist some non-Faradaic processes, such as the Cannizzaro reaction that can also produce HMFCA, which makes partial FE higher than 100%.To clarify this, the following paragraph is added to the manuscript (Page 9, line 213-218): As far as the Faradaic efficiency is concerned, it is high at around 100%.Notably, there is a partial Faradaic efficiency even higher than 100% while counting errors (Figure 1h).The main reason lies in the non-Faradaic process of the Cannizzaro reaction, which can also produce HMFCA and H2 without applying potential.However, through the measurement of the non-Faradaic part of HMFCA (Figure S4), it can be ruled out from the total measured HMFCA.
In addition, the following revision is made in the section of Product analysis (Page

23, line 525-529):
In this electrochemical system, there are two Faradaic processes, i.e., HMFMD 2˗ and HMFMD ˗ oxidation pathways.However, some non-Faradaic processes, such as the Cannizzaro reaction, can also produce HMFCA.Considering non-Faradaic processes, an experiment without applied potential is conducted to measure and rule out this part of HMFCA.

3.
How do the authors distinguish the FE of HMFMD 2-and HMFMD -oxidation pathways?Is it by hydrogen production rate?
Reply: Thanks for the comment.These two reactions are distinguished by the amount of H2 produced.To clarify it, the following revision is added to the manuscript (Page 10, line 218-221): Thus, based on the observation of the generated hydrogen amount, the corresponding Faradaic efficiencies of the DHMFM − and DHMFM 2− oxidation pathways can be determined.Figure 1i shows that the DHMFM − oxidation is the major pathway.

4.
The vibration peaks of DHMFM 2--Au13 (1024 cm -1 ) are also present in Figure 3a-b, so the oxidation of HMFMD 2-still exists at the potential of 0.53-0.93V. Therefore, the absence of H2 production at 0.53-0.93V may be attributed to the higher potential and cannot be fully considered as the effect of intermediates.
Reply: Thanks for the comment.We checked the attribution of the vibration peak in 1024 cm -1 with our calculation result and experimental result in the literature (J.Raman Spectrosc.2011,42, 2069-2076).The peak of 1024 cm -1 can be attributed to the C-O stretching vibration of the hydroxy group (-CH2OH) 1 not only for DHMFM 2-but also for HMFCA.Its existence at the potential of 0.53-0.93V is mainly attributed to HMFCA because the difference between DHMFM 2-and HMFCA can be clearly distinguished by the characteristic Au-O vibration peaks.As a result, the absence of H2 production at 0.53-0.93V should be attributed to the effect of intermediates.To clarify it, the assignment of 1024 cm −1 is revised in Table S1, and the following revision is made in the manuscript (Page 12, line 278-280): The peak at 945 cm −1 belongs to the C-O vibration of DHMFM−-Au13, whereas the peak at 1024 cm −1 is ascribed to the C-O vibration of DHMFM 2− -Au13 and HMFCA.

5.
In page 9, line 203, the oxidation peaks are shown in Figure 1d, not Figure 1c.
Furthermore, there are many mistakes in detail; please check the whole manuscript carefully.
Reply: Thanks for the comment.We have corrected this error by changing "Figure 1c" to "Figure 1d".In addition, we also checked the entire manuscript again and changed "m/s" to "m/z" 6.It is recommended to add in situ IR to make the results more convincing.
Reply: Thanks for the good suggestion.We conducted an in-situ attenuated total reflectance Infrared (ATR-IR) experiment with the Ni-Au electrode.The results (Figure 3) show that HMFMD 2− and HMFMD − intermediates can also be observed.
And the electrochemical behaviors of these two intermediates also correspond with the electrochemical experiment and in-situ Raman experiment in our manuscript.To clarify it, the following contents have been added to the manuscript (Page 13, line 298-326): In situ ATR-IR spectroscopy is used to confirm the existences of the DHMFM 2− and DHMFM − intermediates and the reaction mechanism under the same condition.As shown in Figure 3a, the ATR-IR spectroscopy contains two types of peaks when compared to the baseline: positive and negative peaks.A positive peak indicates production, whereas a negative number indicates consumption.The peak at 1207 cm −1 is attributed to the C-H rocking of HMFCA, [34] which appears from 0.03 V to 0.93 V, indicating HMFCA is produced at this potential.In the range of 0.03-0.63V, it appears as positive peaks indicating the production of HMFCA, which is due to oxidation from HMFMD 2− and HMFMD − .However, in the range of 0.73-0.93V, it appears as negative peaks, indicating the consumption of HMFCA, i.e.HMFCA being further oxidized.Additionally, a positive peak is seen at 1621 cm −1 between 0.43 and 0.93 V, which is attributable to FDCA's C=O stretching. [35]It is possible to infer from the information provided by both of those signals that HMFCA is oxidized to produce FDCA.Furthermore, a small negative peak of 1477 cm −1 appears at 0.23 V to 0.43 V, which is attributed to the C-H scissoring of HMFMD 2− according to the computational results (Figure 3b).It indicates the oxidation reaction via HMFMD 2− .Moreover, a negative peak of 1571 cm −1 is observed from 0.33 V to 0.93 V, which is attributed to the C=C stretching of HMFMD − according to the computational results (Figure 3c).It indicates an oxidation reaction path via HMFMD − .In addition, some negative peaks at 1372, 1522, and 1662 cm −1 are observed from 0.33 V to 0.93 V, which are attributed to the C-H wagging, C=C stretching, and C=O stretching of HMF, respectively.According to the aforementioned description, it can be concluded that the results obtained by in situ ATR-IR are in good agreement with the results obtained by in situ Raman.
In addition, the method for in situ ATR-IR experiment is added to manuscript in the "In situ ATR-IR experiment" part as follows (Page 24, line 572-582): A Thermo Nicolet 8700 spectrometer equipped with an MCT detector cooled by liquid nitrogen is employed for the electrochemical ATR-IR (Otto) measurements.A Si prism is used as the internal reflection element.The Ni-Au electrode is used as the working electrode, Hg/HgO as the reference, which is introduced near the working electrode via a Luggin capillary, and a Pt mesh (1 cm × 1 cm) serves as the counter electrode.1 M KOH with 0.05 M HMF is used as the electrolyte, and the distance between the working electrode and the Si prism is about 1 μm.All spectra are shown in

=
, with Es and ER representing the sample and reference spectra, respectively.
The spectral resolution is 4 cm -1 for all the measurements, unless otherwise mentioned.

Reviewer #2 (Remarks to the Author):
In this manuscript, the authors studied the mechanism of 5-Hydroxymethylfurfural electrooxidation on gold for anodic hydrogen generation with Raman, GC, HPLC, DFT as well as DEMS.A lot of work has been done.However, there are some flaws in the manuscript as shown below.I would suggest reconsidering to publish the manuscript after a major revision.
Reply: Thanks for the valuable comments and suggestions.Based on the suggestions, we have carefully and extensively revised the manuscript, and corrected the flaws.In addition, a series of experiments, such as in situ IR and Raman, are conducted and the results convincingly prove the proposed mechanisms and further confirm the conclusion.

7.
For the DEMS measurements, I did not find the experiment's details in the manuscript.
Reply: Thanks for the comments.We added experimental details of DEMS measurement in the methods sections as follows (Page 24, line 555-561): An in situ DEMS set-up (Shanghai Linglu Instrument Equipment) is employed for the measurement, with a Teflon film separating the electrolyte from the vacuum system to minimize aqueous solvents entering the mass spectrometer.The vacuum system consists of two dry pumps and one turbo pump, and the vacuum is maintained below 2 × 10 −4 Pa.The preparation process for the working electrode is the same as for the Au nanocone array electrode.
8. Lines 297-303, typos, the mass charge ratio should be m/z or m/e instead of m/s.
Reply: Thanks for the comment.We are sorry for the typo.We have carefully checked our manuscript again and changed "m/s" to "m/z".

Figures 2c, S9 and S10a
, the authors claimed that 0.33 V vs 0.17 V (vs.RHE) was used.However, in both Figures S9 and S10, the applied potentials were the same between 0.37 and 0.13 V, but the mass spectra signals were quite different.At positive potentials, there should be positive currents for oxidation reactions.However, the currents showed in Figures were negative.Did you purge solutions with N2 or Ar?
Reply: We are sorry for these mistakes.We double checked the original data and found that we applied a different potential between 0.13 V and -0.37 V, this is a potential range where the HER is able to occur.The Figure S13 (i.e.old Figure S10) is corrected as below: 10.Lines 301-303, at 0.17 V (vs.RHE), the hydrogen evolution reaction on gold cannot happen at this positive potential.
Reply: Thank you for commenting.We sincerely apologize for this mistake.As previously stated, the correct potential values for the HER reaction should be -0.37V (vs.RHE).The manuscript has been revised to correct the mistake.
11. Again, in Figure S1, the currents were negative in both scan directions.You might have had oxygen in the solution.
Reply: Thank you for commenting.We repeated the CV test by purging the electrolyte with Ar to remove the oxygen in the solution, and the result indicates no oxygen in the solution this time.12.It is well known that aldehyde electrooxidation on Ib metals such as Au, Ag and Cu in alkaline media generates hydrogen, and has been studied for many decades.There are many relevant papers that may be helpful for mechanism study, however, were not cited by the authors.

Reviewer #3 (Remarks to the Author):
In the submitted manuscript, the authors attempted to decipher the anodic hydrogen production mechanism during HMF electro-oxidation to HMFCA on gold in basic solutions.In such conditions, HMF, DHMFM, DHMFM -, and DHMFM 2-are present in the bulk state.Previous studies suggested that the anodic H2 production probably originated from the aldehyde group.In this study, anodic H2 production is directly linked to the DHMFM 2-, and specifically the Au-H bond formation at low overpotentials.Also, the DHMFM -↔ DHMFM 2-equilibrium dictates the extent of anodic H2 production.The proposed mechanism is reasonable but I would not consider the level of experimental and theoretical support fully sufficient as this is the main driving point of the paper.Several suggestions to improve and questions are below.
Reply: Thanks for the comments and suggestions.Some additional results of in situ Reply: Thanks for the comment.This is a significant topic.It is frequently crucial for a catalytic reaction to have facet selectivity.We take this issue seriously for this system as well.Different crystal planes, however, were shown to have minimal impact in practical trials.We discovered, after extensive literature study, that Nørskov et al.
investigated the activity of the crystal face of Au in detail.The low-coordinated Au, i.e.
the Au in the edges or the corners, is the one that is active rather than the ideal Au crystal face, which is inactive.In this article, our simulation modeling is likewise built on this basis.In order to imitate low-coordination Au, we constructed a model of Au13.To clarify it, the following revision is made in the revised manuscript (Page 10, line 232-

236):
Since the previous extensive studies by Nørskov et al. show that the well-crystalline Au surface is often inert while the low-coordinated Au, such as edge sites or corner sites exhibit high activity, an Au13 cluster is used to simulate the low-coordinated Au surface for modeling the adsorption of HMF and their intermediates on the Au surface.
14: Could the Raman spectra and hypothesis be verified with metals like Au, Ag, and Pd which have recently been shown to be close to 100% selective for the anodic pathway that produces H2?I would assume then, if the mechanism proposed in the paper is correct, that the DHMFM 2-would be the main intermediate detected in the Raman spectrum.Luckily these materials are also SERS active.This would also help to determine if the proposed insights are generalizable.
Reply: Thanks for the comments and suggestion.To verify the proposed mechanism, a Cu nano-structure electrode is synthesized and an in-situ Raman spectroscopy experiment is conducted since copper is the best catalyst for aldehyde hydrogen evolution reactions with approximate 100% selectivity and it is also reported as the surface enhancing Raman spectra material.It was found that The Raman spectra and proposed mechanisms can indeed be verified by Cu.The following contents have been added to the manuscript (Page 19, line 426-430): To confirm the validity of this mechanism, Cu is employed as the electrocatalyst and in situ Raman spectroscopy is used to analyze the reaction.The results further support our proposed mechanisms.Figures S17 and S18 illustrate the comprehensive results, while the detailed analysis is provided in the Supplementary information.
In addition, the synthesis method for the Cu nano-structure is added in the manuscript as follows (Page 21, line 482-489):

Preparation of the pure Cu nano structured electrode
A piece of copper foam is cut to a size of 1 x 1 cm 2 and washed with ethanol and DI water for 5 minutes, respectively.Then, the copper foam is immersed in a static 2 M NaOH/ 0.11 M APS aqueous solution for chemical oxidation to form a Cu(OH)2 nanoneedle on the surface of the copper foam.After that, the copper foam is immersed in 1 M NaBH4 for 10 minutes to reduce Cu(OH)2 into Cu.Finally, the copper foam is taken out, washed with DI water for 15 minutes, and dried at room temperature for further use.process and Faradaic process ranges from 7.9% (0.33V) to 0.4% (0.93V) depending on the potential.To clarify it, an additional statement is added in the manuscript (page 23, lines 529-530): The ratio of the HMFCA generated by the non-Faradaic process to the Faradaic process ranges from 7.9% (0.33V) to 0.4% (0.93V) depending on potentials.9. Typos -please fix throughout text (e.g.Faradic vs Faradaic) Reply: for the comment.We double checked the full manuscript.The typos have been fixed.

1.
Figure 1.(a) SEM image of Au-Ni electrode.(b) XRD of Ni foam and Au-Ni electrode.(c) XPS spectra of Au-Ni electrode (d) CV curve of Au-Ni foam and Ni foam electrode in 1 M KOH with 0.5 M HMF.(e) CV curve of Au-Ni electrode in 1.0 M KOH with and without 0.5 M HMF, HMFCA, FFCA and FDCA.(f) CV curve of Au-Ni electrode in 0.01M, 0.1M and 1.0M (g) Product yield of FDCA, FFCA, DFF, HMFCA and H2 at various potentials with the same charge.(h) Faradic efficiency and HMFCA selectivity at various potentials of 0.33−0.93V.(i) Faradic efficiency of two pathways at various potentials of 0.33−0.93V.The potentials reported in this study are referred to RHE unless specified otherwise.

Figure
Figure S1 (a-b) SEM images of an Au-Ni electrode (c) TEM image of the Au-Ni electrode surface, and (d) Fourier transform image of the selected area (white square area) in the TEM image.

Figure 3 .
Figure 3. (a) In-situ ATR-IR spectra of HMF oxidation reactions on gold.(b) C-H scissoring of HMFMD 2-, (c) C=C scissoring of HMFMD -, (d) C-H wagging of HMF, (e) C=C stretching of HMF, (f) C-H rocking of HMFCA and (g) C=O stretching of FDCA based on the computational IR frequency analysis.

Figure
Figure S13 (a) Potential signal and (b) Differential Electrochemical Mass Spectrometry (DEMS) signal of hydrogen evolution reaction on Au electrode in 1 M KOD and 0.5 M HMF.
Figure S2 (i.e.old Figure S1) illustrates the new results.

Figure
Figure S2 (a) CV curve of Ni-Au electrode in 1 M KOH.(b) Linear fitting of the Cdl for Au electrode.
IR and Raman are included to support the proposed mechanisms.In addition, the proposed mechanism is validated by a Cu-catalyzed reaction.All of these additional results reinforce the conclusion.13.Facet dependence -how does the surface facet influence the Au-O modes and positions in the Raman spectra.Would this also cause different Au-O vibrations as well as different intermediates?
The following contents have been added in the SI (Page 20-22 of SI):As shown in FigureS17a and b, the surface of the Cu foam is covered by Cu nanoneedles with a diameter of 200 nm.Such a nanostructure with a large surface area is beneficial to both electrocatalysis activity and the surface-enhancing Raman effect.

Figure
FigureS17cshows the XRD pattern of the Cu electrode, in which the peaks at 43.3°, 50.4°, and 74.0° are assigned to Cu (PDF#04-0836).Figure17dshows the electrochemical behavior of the Cu nanostructure electrode in 1M KOH and 0.05 M HMF.It clearly shows that the HMF hydrogen production reaction occurs at 0-0.5 V, whereas the partial Cu is oxidized to Cu2O at 0.5-0.6V.However, both Cu and Cu2O are oxidized to Cu(OH)2 at a potential above 0.6 V.An in-situ Raman experiment is conducted on the Cu nano-structure electrode, and the results are shown in FigureS18, which is different from the spectra conducted on the gold nano-structure electrode.The main difference lies in the signals for the intermediates HMFDM 2-and HMFDM -.In the in-situ Raman experiment for the Cu nano-structure electrode, the peak at 400 cm -1 is attributed to the Cu-O of HMFDM 2- absorbed species.The other typical Cu-O peaks, which have been widely studied, can be assigned to Cu2O (150, 220, 415, 520, 630 cm -1 ), CuO (303, 350, 636 cm -1 ), Cu(OH)2 (292, 488 cm -1 ) and CuO2 -(636 cm -1 ).(ACS Catal.2016, 6, 2473-2481) However, there is no Cu-O peak for HMFDM -observed, indicating that no HMFDM -pathway occurs on the surface of the Cu electrode.The observation is in line with the performance results.In addition, The Cu-H peak, which is considered the most important intermediate for the production of H2, is also observed at 2066 cm -1 .When the potential rises to 0.33V-0.43V,some new peaks appear, and the new peak at 558 cm -1 is attributed to the Cu-O of HMFCA absorbed species.Notably, the intensities of HMFDM 2− and HMFCA peaks decrease since the Cu electrode is oxidized to Cu2O above 0.53 V and

Figure S17 .
Figure S17.SEM images of as synthesis Cu nanostructure electrode in Low (a) and High (b) magnification.(c) The XRD pattern of Cu nanostructure electrode.(d) The LSV curve of Cu nanostructure electrode in 1 M KOH and 0.05 M HMF.

Figure S18 .
Figure S18.In situ Raman spectrum of Cu nanostructure electrode in 1 M KOH and 0.05 M HMF

Figure S3 .
Figure S3.SEM images of as synthesis Au nanostructure electrode in low (a) and high (b) magnification.(c) The XRD pattern of Cu nanostructure electrode.(d) The LSV curve of Cu nanostructure electrode in 1 M KOH and 0.05 M HMF.

Figure S4 .
Figure S4.(a) GC curve from 1 M KOH and 0.05 M HMF without applied potential.(b) Yield of HMFCA and H2 in 1 M KOH and 0.05 M HMF without applied potential.

Figure 3 .
Figure 3. (a) In-situ ATR-IR spectra of HMF oxidation reactions on gold.(b) C-H scissoring of HMFMD2-, (c) C=C scissoring of HMFMD -, (d) C-H wagging of HMF, (e) C=C stretching of HMF, (f) C-H rocking of HMFCA and (g) C=O stretching of FDCA based on the computational IR frequency analysis.

Figure 1 .
Figure 1.(a) SEM image of Au-Ni electrode.(b) XRD of Ni foam and Au-Ni electrode.(c) XPS spectra of Au-Ni electrode (d) CV curve of Au-Ni foam and Ni foam electrode in 1 M KOH with 0.5 M HMF.(e) CV curve of Au-Ni electrode in 1.0 M KOH with and without 0.5 M HMF, HMFCA, FFCA and FDCA.(f) CV curve of Au-Ni electrode in 0.01M, 0.1M and 1.0M (g) Product yield of FDCA, FFCA, DFF, HMFCA and H2 at various potentials with the same charge.(h) Faradaic efficiency and HMFCA selectivity at various potentials of 0.33−0.93V.(i) Faradaic efficiency of two pathways at various